The present invention relates to a nonvolatile semiconductor memory having a floating gate electrode, and being operable to electrically write data and to erase the once written data by means of ultraviolet ray radiation.
Such a type of conventional memory includes an electrically programmable read only memory (EPROM) of p-channel type having floating gate, which is disclosed in Japanese Patent of Bentchkowsky published in 1976 (Japanese Patent Publication No. 31073/1976). This patent discloses the EPROM of floating gate type comprised of a pair of source and drain regions of the P.sup.+ -type formed on a surface of N-type silicon substrate, a floating gate electrode formed thereon through a gate insulating film of 500 .ANG. to 1000 .ANG., and a silicon oxide layer covering the electrode. The floating gate electrode is composed of P-type silicon.
FIG. 2 shows one example of the conventional EPROM element of floating gate type. A source region 12 and drain region 13 of the P-type are formed in a surface of N-type silicon substrate 11. A gate oxide film 16 is formed on the substrate surface over a channel defined between the source region 12 and the drain region 13 at a thickness of 500 .ANG. to 1000 .ANG.. A floating gate electrode 14 composed of P-type polysilicon is disposed on the channel through the gate oxide film 16. An insulating layer 15 composed of insulating material such as silicon oxide is formed over and around the floating gate electrode 14 so as to electrically insulate the same from other regions.
According to the conventional technology, the floating gate electrode is composed of P-type silicon, and the gate insulating film has the thickness of 500 .ANG. to 1000 .ANG.. However, the recent improvement in the semiconductor fabrication technology brings about remarkable micronization of semiconductor integrated circuits. In order to effect micronization of a MOS element by scaling, a gate insulating film is needed to have the thickness of 100 .ANG. to 500 .ANG.. When the thickness of the gate insulating film is reduced below 500 .ANG., the gate electrode cannot be composed of P-type silicon. The reason is that the gate electrode of P-type silicon is normally formed by doping or diffusing boron into polysilicon grown in a gas phase, and the boron particles would easily pass through a gate insulating film of thermally oxidized silicon of less than 500 .ANG. due to thermal diffusion to thereby reach to and diffuse into the silicon substrate. The boron diffused into the silicon substrate causes a change of the threshold voltage of the MOS transistor. Further due to excessive diffusion of boron particles, the thermally oxidized film is degradated to exhibit low dielectric strength and low reliability.